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r r Page 5 r Ms Margaret Lagono February 21, 1997 rbenzene-impacted samples indicating that anaerobic activity is occurring as well Biodegradation indications using alkalinity and ferrous iron data are inconclusive ' Ferrous iron was not detected in any groundwater sample CONCLUSIONS ' The extent of petroleum-impacted groundwater has been delineated, and is contained solely within the Earthgrarn's property boundary Current TVH-G and BTEX concentrations measured in samples collected during this quarterly monitoring event are ' consistent with a decreasing trend of petroleum impact observed at this site From 1992 to the present, TVH-G concentrations have decreased almost four-fold, and benzene concentrations by two to six times in samples collected from Morutonng Well MW-1 Biodegradation assessment data collected during this quarter suggests that the significant ' petroleum concentration decreases noted at this site are a result of source removal, and natural aerobic and anaerobic microbial activity ' RECOMMENDATIONS As you requested in your July 16, 1996 letter to Earthgrarns, Philip evaluated the relative cost effectiveness of"actively remediating the source area to reduce contaminant Ievels compared to monitoring over a long period of time" Based on the biodegradation ' progress and rates observed, Philip concludes that natural attenuation and passive monitoring is significantly more cost effective than the design, installation, operation and maintenance of an active groundwater sparge and soil vent remediation system Implementation of such a system for a period of at least four years would typically cost in r excess of$150,000 Passive monitoring for the same time period would generally cost less than a third of that amount In accordance with those cost differentials, and current State Water Resources Control Board guidance for leaking underground storage tank sites ' with naturally occurring biodegradation activity, Philip recommends continued _ groundwater monitoring at this site in lieu of implementing an engineered remediation system ' Because the extent of petroleum-impacted groundwater at this site has been delineated, and a vast collection of quarterly monitoring data has been collected, Philip further recommends that future groundwater monitoring be performed on a semi-annual basis ' Such a monitoring schedule would be dust as effective as quarterly monitoring in assessing evolving groundwater conditions The next proposed groundwater monitoring event would be performed in June 1997 1 r E 121382112599714gtr96 doc